The present invention relates to a developing device having an image carrier and a developer carrier which face each other to define a developing region therebetween, and developing a latent image electrostatically formed on the image carrier by a developer deposited on the developer carrier. More particularly, the present invention is concerned with a developer carrier incorporated in such a developing device and a method of producing the same.
The above-described type of developing device is applicable to an electrophotographic copier, laser printer, facsimile machine or similar image forming equipment which prints out an image by developing a latent image electrostatically formed on an image carrier. The developing device uses either a two-component developer consisting of a toner and a carrier or a one-component developer which does not include a carrier. The one-component developer may contain an auxiliary agent in addition to a toner, as well known in the art. A prerequisite with the development using such a developer is that the developer be sufficiently charged and fed to the developing region in a required amount in order to form a quality image having a desired density. However, a conventional developing device cannot readily meet the above requirement and is apt to lower the image density. This is especially true when the device uses a one-component developer.
In light of this, there has been proposed a developing device having a developer carrier on which a charge holding layer is formed, and a fur brush roller, sponge roller or similar charging member held in contact with the charge holding layer, as disclosed in Japanese Patent Laid-Open Publication No. 43767/1986. The charge holding layer and the charging member are charged to opposite polarities by friction. A developer charged to a polarity opposite to the polarity of the charge holding layer is electrostatically deposited on the layer and transported to the developing region. With this implementation, however, it is impossible to produce sufficiently intense electric fields in the vicinity of the charge holding layer. Hence, it is not easy to deposit a developer great enough in amount to form an image having a high density on the surface of the developer carrier.
The developer carrier may be formed with undulations on the surface thereof, as taught in Japanese Patent Laid-Open Publication No. 53976/1985. This kind of approach is successful in transporting a greater amount of developer to the developing region since the developer will be filled in the undulations. However, the problem is that a substantial amount of the developer being transported is not sufficiently charged and is, therefore, apt to degrade the image quality.